1. Field of the Invention
This invention relates generally to epitaxial reactors for producing uniform film coatings on selected surfaces and more particularly to a barrel epitaxial reactor, which has a greater batch capacity than prior art barrel epitaxial reactors and which produces an epitaxial layer quality at least as good as that produced by the prior art barrel epitaxial reactors.
2. Description of the Prior Art
Several alternative types of radiant heating epitaxial reactors are known in the prior art. See, for an example of one type of epitaxial reactor, U.S. Pat. No. 4,081,313 issued on Mar. 28, 1978 to McNeilly et al. Generally, each epitaxial reactor has (i) a reaction chamber, (ii) a heat source and temperature control, and (iii) gas sources and a gas flow controller.
A side cross sectional view of one prior art barrel epitaxial reactor is shown in FIG. 1A. Reactor 10 is available from Applied Materials of Santa Clara, Calif. Reactor 10 is supplied by Applied Materials under Model Nos. 7600, 7800, 7810 and 7820.
Reaction chamber 60 of reactor 10 is the interior volume of bell jar 40 between transition region 46 and edge 45A of bead-blasted region 45. Heat source 50 consists of five banks of quartz halogen lamps 51. Each bank of lamps 51 consists of a column of fourteen lamps 51, as described more completely below. Gas ring 20 is connected to the gas sources and a gas controller. Heat source 50 and reactor chamber 60 are contained within housing 15. Housing 15 of reactor 10 bounds a volume which is about 36 inches high and about 46 inches wide.
The five banks of lamps 51 in heat source 50 form a pentagon about bell jar 40, as illustrated in FIG. 1B, with each bank occupying one side of the pentagon. In FIG. 1B, only the relative positions of the five banks of heat source 50 and bell jar 40 are represented for clarity. (The figures are not drawn to scale and are intended to only show the relative positions of components within reactor 10.) Lamps 51 produce radiant energy in the short wavelength range, i.e., approximately one micron or below. Each lamp 51 is mounted in a one and one-eighth inch parabolic gold plated highly polished reflector 52. (In the Figures, subscripts on reference numerals are used to (i) represent similar components and (ii) to denote particular features of a component. In the description, a reference numeral without a subscript is used as a shorthand notation to refer to all occurrences of that reference numeral with subscripts in the drawings).
The short wavelength radiant energy from heat source 50 is transmitted through transparent quartz wall 41 of bell jar 40. Quartz wall 41 absorbs little or no radiation. Radiant energy from heat source 50 is incident on a susceptor 65 mounted within reaction chamber 60. Susceptor 65 is suspended in bell jar 40 by a quartz hanger 61 which in turn is rotatably connected through opaque quartz top flange 47 to a rotation means (not shown) so that susceptor 65 and wafers 70 mounted thereon can be rotated relative to heat source 50. The rotation of susceptor 65 ensures uniform heating of susceptor 65 and wafers 70.
Susceptor 65 is made from a material that absorbs the radiant energy from heat source 50 and provides a uniform temperature surface for wafers 70. Susceptor 65 is usually made of graphite and coated with a thin coating of silicon carbide over the outer surface. The silicon carbide coating prevents contamination of wafers 70 with carbon.
The size of susceptor 65 limits the capacity of reactor 10 because the size determines the number of wafers, i.e., the batch capacity, which can be placed in reaction chamber 60 at one time. The "batch capacity" is sometimes referred to as the "batch size". Table 1 lists the batch capacity of barrel epitaxial reactor 10.
TABLE 1 ______________________________________ Batch Processing Capacity of Barrel Epitaxial Reactor 10 Wafer Diameter (mm) Batch Size ______________________________________ 100 24 125 12 150 10 200 4 ______________________________________
Susceptor 65 has a length of approximately 16.75 inches and provides a flat zone of approximately 12 inches. As used herein, the length of susceptor 65 refers to the vertical dimension of susceptor 65. The vertical dimension of susceptor 65 or of any other component in reactor 10 is the dimension in the same direction as the direction extending from the gas entrance of reaction chamber 60 (the top of reaction chamber 60) to the gas exit of reactor chamber 60 (the bottom of reaction chamber 60).
The "flat zone" of susceptor 65 is the region on each face of the exterior surface of susceptor 65 where wafers may be placed and uniform epitaxial depositions achieved on the wafers. Flat zone 67 of susceptor 65 is illustrated in FIG. 1 by the dashed line which encloses wafers 70.
Another criteria used to measure the capacity of the barrel reactor is the percentage of the surface area of the susceptor covered by the wafers being processed. For reactor 10, a six-sided susceptor is used to process 125 mm diameter wafers Two wafers are placed on each face of the susceptor to give the twelve wafer batch size in Table 1. The area of one face of the susceptor is about 86.645 in.sup.2 (559.0 cm.sup.2). The area in contact with the susceptor of two 125 mm diameter wafers is about 38.04 in.sup.2 (245.44 cm.sup.2). Hence, the percentage of a susceptor face covered by the wafers is 38.04.div.86.645 or about 44%.
For processing of 200 mm diameter wafers, a four sided susceptor with an area of about 134.370 in.sup.2 (866.90 cm.sup.2) per face is used. Each face of the susceptor holds one wafer with an area of 48.7 in.sup.2 (314.16 cm.sup.2). Thus, for 200 mm diameter wafers the percentage of a susceptor face covered by the wafer is 48.7.div.134.370 or about 36%.
For processing of 100 mm diameter wafers, an eight sided susceptor with an area of about 78.02 in.sup.2 (503.35 cm.sup.2) per face is used. Each face of the susceptor holds three wafers. Each wafer has an area of about 12.2 in.sup.2 (78.54 cm.sup.2). Thus, for 100 mm diameter wafers the percentage of a susceptor face covered by wafers is about (12.2.times.3).div.78.02 or about 47%.
For processing of 150 mm diameter wafers, a five sided susceptor with an area of about 116.09 in.sup.2 (748.97 cm.sup.2) per face is used. Each face of the susceptor holds two wafers. Each wafer has an area of about 27.4 in.sup.2 (176.71 cm.sup.2). Thus, for 100 mm diameter wafers the percentage of a susceptor face covered by the wafers is about (27.4.times.2).div.116.09 or about 47%.
For reactor 10 (FIG. 1A), flat zone 67 is defined so that the variation in thickness of the epitaxial layers from wafer to wafer on adjacent wafers is at most .+-.5% and the variation in resistivity of the epitaxial layers from wafer to wafer on adjacent wafers is at most .+-.5-10%. Across flat zone 67, the thickness variation of the epitaxial layers from wafer to wafer on any two non-adjacent wafers, including wafers at opposite ends of the flat zone, is at most .+-.4-7% and the resistivity variation from wafer to wafer on any two non-adjacent wafers is at most .+-.4-12%. These variations are the industry standards for growth of epitaxial layers.
Quartz bell jar 40, in which susceptor 65 is mounted, has a lower flange 42 which is connected to a lower support 43 for bell jar 40. O-ring 24 provides a seal between lower flange 42 and exhaust cup 30. Lower support 43, about 3 inches in length, is connected to side wall 41 by lower curved portion 44 of jar 40. Lower curved portion 44 has been bead-blasted (i.e., subjected to a high velocity stream of beads, typically of 180 grit glass beads) to produce an opaque surface region 45. Edge 45A is the end of bead-blasted lower curved portion 44. As previously described, edge 45A defines the bottom of reaction chamber 60. Transparent vertical sidewall 41 of quartz bell jar 40 terminates in a transition region 46 to which opaque quartz top flange 47, typically made of white quartz, is connected. Transition region 46 is about three inches in length.
The spatial relationship between side wall 41, curved portion 44, lower support 43 and transition region 46 affects the gas flow dynamics, e.g., the gas velocity, the gas mass flux, the flow mixing, and the flow turbulence within reaction chamber 60. If the spatial relationship of these portions of bell jar 40 is modified, the gas flow dynamics within reaction chamber 60 are altered. Since the uniform growth of the epitaxial layers in reaction chamber 60 is directly dependent upon the gas flow dynamics, any modification to bell jar 40 affects the uniformity of the epitaxial layers grown.
Opaque top flange 47 rests on a base plate 32 and is sealed by an O-ring 21 between base plate 32 and top flange 47. Top flange 47 is secured by a gas ring 20 and two O-rings 22, 23. The reactant gases are introduced into reaction chamber 60 through gas ring 20. After the gases pass through reaction chamber 60, the gases are exhausted through an opening or openings in lower flange 42 into stainless steel exhaust cup 30. Exhaust cup 30 is attached to support assembly 31 which positions bell jar 40 within epitaxial reactor 10.
The gas flow through reaction chamber 60 depends upon the desired epitaxial growth rate and the process specifications. In reactor 10, the epitaxial growth is a function of the hydrogen flow and the flow of the other reactant gases through chamber 60. The gas flow into reaction chamber 60 is controlled using jet settings that are adjusted to separate points on a grid and control of hydrogen main flow and hydrogen rotation flow. The control of jet settings and hydrogen flow is known by one skilled in the art. For reactor 10, typical jet settings on the grid for uniform epitaxial layer growth are 3.5 and 3.5 and a typical hydrogen flow setting for uniform epitaxial layer growth is a main flow of 120 liters per minute (l/m) and a rotation flow of 100 l/m.
Epitaxial growth on wafers 70 within reactor 10 requires temperatures within the range of about 900.degree. to 1200.degree. C. However, quartz wall 41 is typically maintained at about 600.degree. C., so that relative to the 900.degree.-1200.degree. C. temperatures in reaction chamber 60, wall 41 is cold. Cold wall 41 of bell jar 40 limits growth of films on the interior surface of wall 41. As used herein, the "interior" surface and "exterior" surface of a bell jar are relative to reaction chamber 60, so that the "interior" surface is a boundary of reaction chamber 60 and the "exterior" surface is outside of reaction chamber 60.
Wall 41 must be maintained at a constant temperature, because if the temperature of wall 41 increases, films are deposited on wall 41. These films absorb radiant energy which in turn affects the uniformity of epitaxial layers grown in reactor 10.
To maintain wall 41 of bell jar 40 at the desired temperature and to protect the other components in reactor 10 from high temperatures, heat source 50 and the exterior surface of jar 40, i.e., transition region 46, wall 41 and curved portion 44 are cooled by forced air circulation. Specifically, a blower 80 provides a constant airflow to a supply plenum 81. Supply plenum 81 has a vertical riser 85 which is connected to water cooled heat exchanger 53 on the back of lamp assemblies 50. Thus, air from blower 80 enters supply plenum 81 and is passed through heat exchanger 53 on the back of lamp assemblies 50 through reflector 52 and around lamps 51 into region 62 between bell jar wall 41 and lamp assemblies 50.
Since there is a resistance to airflow from vertical riser 85 into region 62, some of the airflow from blower 80 is diverted from supply plenum 81 through two inch outer diameter pipe 82 into upper plenum region 63. The forced air flow from pipe 82 through upper plenum region 63 prevents hot stagnant air from accumulating in plenum region 63.
Forced air flow from region 63 and from vertical riser 85 flows down sidewall 41 of bell jar 40 into exhaust plenum 83, which is located at the bottom of reactor 10. The air passes through exhaust plenum 83 into a heat exchanger 84. Heat exchanger 84 is water cooled with a flow of about 5 gallons per minute. The cooling water flow is regulated by flow restrictor 94. Thus, the air is cooled as it flows through heat exchanger 84 and the cooled air from heat exchanger 84 flows into blower supply plenum 86.
In addition to the air cooling, upper plenum region 63 has a water cooled cooling plate 88 which has a gold plated surface adjacent to lamp assemblies 50. Similarly, supply plenum 81 is cooled by water cooled wall 89 which has a gold plated surface facing bell jar 40 that reflects radiant heat energy away from supply plenum 81. In addition, a gold plated protective collar 87 is mounted about lower flange 42 of bell jar 40. The gold plating on these components reflects radiant energy away from other components exterior to reaction chamber 60, and hence limits both heating losses from reaction chamber 60 and heating of components exterior to reaction chamber 60.
The forced air cooling of reactor 10 maintains a uniform wall temperature for the configuration of bell jar 40, heat source 50, supply, exhaust and upper plenums 81, 82, 63 shown in FIG. 1A. If the configuration of these components is changed, the forced air cooling of wall 41 and consequently the wall temperature is affected because the air flow over wall 41 would be changed. As the wall temperature increases, deposits start to grow on the wall which in turn affect the gas distribution within reaction chamber 60. Further, such a change in wall temperature modifies the temperature profile in the reaction chamber 60 which in turn can further affect the gas distribution within reaction chamber 60. The cumulative effect of such changes on the uniform growth of epitaxial layers is unknown.
Each of the five banks of heat source 50 has fourteen lamps 51 (FIG. 2A). The radiant energy from each lamp 51 is directly proportional to the voltage across the lamp. Therefore, as illustrated in Table 2, a higher voltage is applied to lamps 51.sub.1 -51.sub.3 and 51.sub.12 -51.sub.14 on the upper and lower periphery respectively of reactor chamber 60. Outermost lamps 51.sub.1, 51.sub.14 (FIG. 2A) of each bank have an applied voltage of 350 volts, while the next two lamps 51.sub.2, 51.sub.3 and 51.sub.12, 51.sub.13 each have an applied voltage of 300 volts. The remaining eight lamps 51.sub.4 -51.sub.11 each have an applied voltage of 240 volts.
TABLE 2 ______________________________________ Voltage Distribution for Each Bank of Heat Source 50 Lamp No. Voltage Lamp No. Voltage ______________________________________ 51.sub.1 350 51.sub.8 240 51.sub.2 300 51.sub.9 240 51.sub.3 300 51.sub.10 240 51.sub.4 240 51.sub.11 240 51.sub.5 240 51.sub.12 300 51.sub.6 240 51.sub.13 300 51.sub.7 240 51.sub.14 350 ______________________________________
The higher voltages are required for six outermost lamps 51.sub.1 -51.sub.3, 51.sub.12 -51.sub.14 in each bank to compensate for the boundary conditions, i.e., the energy absorbing structure at the top and bottom of reactor 10, and heat losses from susceptor 65 and reaction chamber 60.
Lamps 51 in each bank are separated into three groups: a first group consisting of lamps 51.sub.1 -51.sub.5 ; a second group consisting of lamps 51.sub.6 -51.sub.9 ; and a third group consisting of lamps 51.sub.10 -51.sub.14. Each group of lamps 51 is connected to a silicon controlled rectifier (SCR). The load on the SCR for the second group is only about 70% of the load on the other two SCRs. Since each lamp bank has fourteen lamps, two fourteen pin connectors are used to connect power to each bank of lamps.
An infrared sensor (not shown) is lowered through a sheath in quartz hanger 61 into susceptor 65 and is positionable at locations 64 in susceptor 65. Each of the three locations permits measurement of the radiant energy from one of the three groups of lamps as defined above. The infrared sensor is connected through a closed loop temperature control system to the power supply for heat source 50 and the voltage across lamps 51 is adjusted so as to obtain about the same temperature at each of three locations 64.sub.1, 64.sub.2, 64.sub.3 on susceptor 65.
To increase wafer 70 throughput of reactor 10 (i.e., the number of wafers which can be processed in each batch of wafers), flat zone 67 must be enlarged. If susceptor 65 is lengthened in the vertical direction to provide a larger flat zone 67, either spacing Y between bottom 65.sub.1 and line 45A must be decreased, or reaction chamber 60 must be lengthened. As spacing Y decreases, the flat zone is moved further into the boundary region of heat source 50. Hence, uniformity of epitaxial layers grown in reactor 10 is affected by a change in spacing Y.
Placing a larger reaction chamber 60 within housing 15 of reactor 10 does not appear possible because the larger chamber necessarily changes the relationship between the cooling apparatus used to maintain the temperature of wall 41, the heat source 50 and the temperature in reaction chamber 60 As previously described, such changes affect the uniform growth of epitaxial layers so that even if the modification is possible, the increased batch capacity may not have epitaxial layers within industry uniformity standards.
If a vertically longer reaction chamber 60 is placed in housing 15, the upper and lower edges of the larger chamber 60 are necessarily closer to housing 15. The exterior of housing 15 is coupled to the ambient temperature of the room in which reactor 10 is contained, while wall 41 is at about 600.degree. C. and susceptor 65 at about 900.degree.-1200.degree. C. Thus, as reaction chamber 60 becomes larger, heat source 50 must maintain the large temperature differential between reaction chamber 60 and the ambient room temperature, but the resistance to heat flow from reaction chamber 60 decreases because reaction chamber 60 is closer to housing 15. Hence, the energy from heat source 50 must compensate for any increased heat losses from chamber 60 that result from the reduced resistance to heat flow from chamber 60. A larger heat source affects the gas flow dynamics within chamber 60 in a manner similar to that previously described for changes in bell jar 40, and consequently the epitaxial layer growth within chamber 60.
Moreover, the larger heat source may adversely affect other components in reactor 10. Reflector 52 behind heat source 50 is gold plated aluminum. Reflector 52 must be maintained at a temperature below the melting point of aluminum. Any increase in heat source 50 may result in overheating of the aluminum, which in turn would cause the plating to be deformed and subsequently introduce nonuniformities in heat source 50. Such nonuniformities would further alter the growth of epitaxial layers.
Reaction chamber 60 geometry (e.g., the size and shape of bell jar 40, the size and shape of susceptor 65, and the position of susceptor 65 within bell jar 40), heat source 50, gas flow, and forced air cooling of reactor 10 were selected to provide a maximum batch size with each wafer in the batch having an epitaxial layer within industry uniformity standards. To increase the batch size of reactor 10 requires changing at least reaction chamber 60 geometry so that more wafers can be placed within reaction chamber 60. In addition, other characteristics of the reactor would probably have to be modified.
Unfortunately, the characteristics of reactor 10 provide no guidance on how to increase the batch capacity of reactor 10 because, as described above, a change in one characteristic of the reactor affects other characteristics of the reactor. The gas flow dynamics in reaction chamber 60, the temperature profile in reaction chamber 60, the forced air cooling, heat losses from reaction chamber 60 and the reflection of radiant energy into and from reaction chamber 60 are all coupled so that a change in any one affects the others.
For example, if the vertical length of susceptor 65 is increased to hold more wafers, the increased length, in addition to the problems discussed above, increases drag on gas flow through reaction chamber 60. The change in gas flow affects the heat carried from the reaction chamber by the gas. In addition, heat source 50 must be increased to maintain the larger susceptor at a uniform temperature. The change in the heat source further affects the gas flow and the heat losses from the reaction chamber.
Therefore, a simple increase in the length of the susceptor alters many conditions in reactor 10 and each of these conditions affects the growth of epitaxial layers. Similarly, each change in reactor geometry, heat source, gas flow, and forced air cooling affects the uniform growth of epitaxial layers in reactor 10. Since the cumulative effect of these changes is unknown, the feasibility of increasing the batch size of reactor 10 is unknown.
As previously noted, reactor 10 was optimized to provide a maximum batch size within industry uniformity standards. Increasing the batch size of reactor 10 necessarily requires a reaction chamber geometry, heat source, and forced air cooling different from those currently used in reactor 10. Therefore, a new reactor which both increases the batch size of reactor 10 and maintains the epitaxial layer uniformity of reactor 10, is needed to improve the performance of reactor 10. As described more completely below, a prior art modification of reactor 10, which increased the batch size by about 50% for one size wafer, resulted in unacceptable variations in the uniformity of the epitaxial layers.
The prior art modification of reactor 10 did not use a larger susceptor, which would introduce the uncertainties discussed above, but rather the modification extended flat zone 67 of susceptor 65 in the vertical direction. Typically, flat zone 67 for wafers 70 is maintained a first selected distance 68 from top edge 65.sub.2 and bottom edge 65.sub.1 of susceptor 65, and a second selected distance 69 from side edges 65.sub.3, 65.sub.4. First and second distances 68, 69 are selected so that the surface temperature of susceptor 65 is nearly constant within flat zone 67.
The prior art modification of reactor 10 increased flat zone 67 of susceptor 65 by decreasing first selected distance 68. Therefore, flat zone 67 was closer to upper and lower edges 65.sub.2, 65.sub.1 of susceptor 65. To maintain the regions of flat zone 67 closest to edges 65.sub.2, 65.sub.1 at about 900.degree. C.-1200.degree. C. required raising the temperature of the susceptor over the increased length of flat zone 67. Therefore, a larger energy input to these regions was required to compensate for heat losses from edges 65.sub.2, 65.sub.1. Further, if the structure in regions immediately above and below the flat zone was heated by supplying more energy to these regions, the temperature differential between the adjacent structure and the flat zone would be lowered. The lower temperature differential would reduce the heat losses from the flat zone and the reduction in heat losses would help maintain a uniform temperature over the larger flat zone of susceptor 65.
Hence, in the prior art modification, heat source 50 (FIG. 2A) was replaced by heat source 50', as illustrated in FIG. 2B. Reflector 52 (FIGS. 1 and 2A), discussed above, has a lip 55 at the top and bottom which is formed at about a 90.degree. angle to the body of reflector 52. In the modified heat source 50', as shown in FIG. 2B, ends 55' of reflector 52' were flared outward and reflector 52' was broken into three segments 52.sub.1 ', 52.sub.2 ', 52.sub.3 '. A one-eighth to one inch spacer 56 was inserted between the segments. The voltage on outer lamps 51.sub.1 ', 51.sub.14 ' was increased to 480 volts and the four lamps 51.sub.2 ', 51.sub.3 ', 51.sub.12 ', 51.sub.13' were operated at 240 volts. The higher voltages on lamps 51.sub.1 ' and 51.sub.14 ' provide a higher radiant energy output so as to compensate for the increased heat losses associated with moving flat zone 67' closer to the periphery of reaction chamber 60. Flared ends 55' of reflector 52' reflected radiant energy over a larger area than reflector 55, which, as described above, should help minimize heat losses from the larger flat zone.
The change in heat source 50 coupled with overlap of wafers on susceptor 65 was intended to increase the flat zone for 125 mm diameter wafers so that the batch size could be increased from 12 wafers to 18 wafers. Hence, the modification was intended to increase the surface area of the susceptor covered by the wafers from 44% to in the range of about 60-65%. However, the variation in resistivity of the epitaxial layer of adjacent wafers was about .+-.30%. The batch size for 100 mm, 150 mm and 200 mm diameter wafers was not changed by this modification.
Accordingly, based on these results, the batch size of reactor 10 for 125 mm diameter can be increased by about fifty percent only if a .+-.30% variation in resistivity of the epitaxial layer of adjacent wafers is acceptable. In addition, the uniformity standards for other wafer sizes must also be relaxed. In general, a .+-.30% variation is not acceptable so that heat source 50' (FIG. 2B) in combination with using a larger area of the susceptor is not a viable method for increasing the batch capacity of reactor 10.
These results demonstrate that increasing the batch capacity is not a simple process of scaling and compensating for anticipated effects of size changes. Rather, as described above, any change in reactor 10 results in a complex set of interactions which affect the uniformity of the epitaxial layer. Further, given the poor results obtained by increasing the energy output of the portion of heat source 50' closest to housing 15, increasing the capacity of reactor 10 by using a larger susceptor, as described above, does not appear feasible because the larger susceptor would appear to require a heat source similar to that illustrated in FIG. 2B. Therefore, the batch capacity of reactor 10, as given in Table 1, appears to be the best that can be achieved while maintaining industry standards for uniformity of the epitaxial layers.